Disposal of solid waste has been a growing problem and has risen to crisis levels in some parts of the country. One solution to the problem is to recycle materials that normally are landfilled. Various materials can be pulled from the waste stream and recycled with one such component being the different plastic materials.
Further, there are other industries that wish to reclaim materials. Industries such as wire & cable and automotive have a multitude of materials that have value and could be reclaimed. A major obstacle to the reclamation of commingled plastic materials is the extreme difficulty to separate them into discrete components.
It is important to note that reusing commingled plastics (i.e., without separation) is not cost effective nor efficient and further substantially reduces the properties of the plastic materials. For example, PET and PVC materials are not compatible. PET melts at about 500.degree. F. while PVC will degrade at approximately 400.degree. F. Upon degrading, PVC gives off hydrochloric acid which destroys the desirable properties of PET material. It should be noted that small amounts (on the order of 1 part in 500) of PVC mixed in with PET will destroy PET when the two materials are melted together.
Each of the noted industries, as well as other applications utilizing plastic materials, uses many different types of plastic materials. As an example but recognizing other industries or markets pertain also, the plastic packaging industry will be focused upon.
Typical rigid household plastic packages (bottles and other type containers) are made of 5 plastic materials. Other plastic materials are used as well, but the 5 highest volume materials are:
PE (Polyethylene; High Density (HDPE) Low Density (LDPE)) PA1 PET (PolyEthylene Terephthalate) PA1 PVC (PolyVinyl Chloride) PA1 PP (PolyPropylene) PA1 PS (Polystyrene). PA1 amorphous PA1 biaxially oriented PA1 crystallized. PA1 a) Positive pole attraction; PA1 b) Negative pole attraction; and PA1 c) "Minglings".
To collect the various plastic materials in an economic manner, all the different material bottles are thrown together and commingled. The bottles are either compacted (baled) or ground (chopped into smaller particles) for space reduction. The most efficient manner would be to grind the whole bottle at the point of collection. Because of the difficulties described in separating the ground particles, this method has not been widely introduced.
Some of the applications for the plastic containers are clear while some are colored (transparent.fwdarw.opaque). An existing method to separate the different materials is to sort by hand. This method is costly and fraught with error as many applications utilize two or more plastic materials. For example, edible oil bottles are made from both PET and PVC materials. This method can only be used with whole bottles and preferably when they are not crushed.
Another method being investigated to separate whole PVC bottles from other plastics is one which uses an energy source to excite the chlorine molecules which are then detected. The bottle is thus identified and ejected from the stream. This method is still experimental but it also has drawbacks in that whole bottles are required, it utilizes radiation sources such as x-rays which pose safety and health hazards and further make the system expensive to operate and maintain.
For ground commingled materials, a more effective manner of separating some of the plastic materials is to separate via the differences in density of the various plastic materials. Generally, this is effective only in separating the materials with densities with&lt;1 g/cc. Materials with densities&lt;1 g/cc generally are PE and PP.
The separation process generally is a float/sink operation where the lighter fractions float on water and are culled off while the heavier fraction sinks to the bottom. The heavy fraction would generally constitute PET, PVC and PS. The densities of these materials are very similar and in the case of PET and PVC they overlap. As an example; typical PVC densities range from 1.25 to 1.36 g/cc while typical PET ranges from 1.32 to 1.39 g/cc. With the overlap in densities, separation via the float/sink method (even using solvents with densities&gt;1 g/cc) will not work.